Spread-spectrum communications is a system using spreading modulation technology where spreading-sequences are modulated by transmit-data. Due to this spreading modulation, a data-sequence spectrum having a relatively narrow bandwidth is spread to a wide frequency-band, producing a spread spectrum signal to be transmitted. In a region (cell or sector) where a base-station (BS) provides communications services, there are users of a plurality of user-stations (hereafter called users). Such a communications system is excellent in that a low transmission power per unit frequency is consumed, disturbance to other communications can be kept at a relatively low level, and the system has inherently strong resistance to jumming noise (AWGN) mixed in a transmission process and inter-station-interference-noise incoming from mobile stations other than a desired station, namely interfering stations. However, since communications from a large number of stations share the same time-slot and the same frequency band, there is a problem in which an increase in the number of users to be accommodated per unit band is impeded by the inter-station-interference(-noise). That is to say, disturbance caused by such noise decreases frequency-utilization-efficiency and increases required transmit-power.
FIG. 16 is a block diagram illustrating the general construction of a mobile communications system which performs direct-sequence spread-spectrum (DS-SS) communications via a radio communications channel. Here, a transmitter TXk of the k-th user uk (k=1, 2, . . . K) among K users in a cell modulates a radio-band carrier-wave with binary transmit-data bk to obtain a Binary Phase Shift Keying (BPSK) symbol skBP, and modulates the k-th spreading-sequence gk among K sequences allocated to K users with BPSK symbol skBP to produce a spread spectrum symbol sk. (symbol denotes a time limited signal conveying data) Thereafter, sk is transmitted through a radio communications channel. In order to discriminate the addresses of the K users, pseudo-noise (PN) sequences each of which is different from one another are used as the k-th sequence gk.
A receiver RX receives through an antenna a receive-symbol r which includes, as the components, spread-spectrum-modulated symbol received from all the users, and demodulates receive-symbol r by a local carrier-wave {circumflex over (f)}0(=f0) to obtain a base-band symbol rBB. Receiver RX applies the base-band symbol to a matched filter MFk matched to the k-th spreading-sequence gk to generate a soft-output {tilde over (b)}k as the k-th soft-output. Soft-output {tilde over (b)}k is compared with a threshold value by a hard decision circuit DEC to obtain the k-th detected value of binary data {circumflex over (b)}k (the k-th means that the data has been sent from the k-th user) (This matched filter detection is called “correlative detection”).
Detected data {circumflex over (b)}k is applied to a synchronizing circuit SYNC. A generating timing of the spreading-sequence is controlled so as to be synchronized with the carrier phase and the component of the k-th user specific received symbol component contained in receive-signal r. In TX and RX in FIG. 16, the arrangement of sequential order of multiplying functions of carrier-wave fC({circumflex over (f)}C) and spreading sequence gk are often exchanged each other. However, the overall modulation and demodulation-functions remain the same, and any configuration may be used.
The above-described receiver uses a reception system where different respective matched filters to detect corresponding user symbols are arranged in parallel. In this system, a cross-correlation between the k-th sequence gk allocated to a user and the k′-th (different) sequence gk′(k≠k′) allocated to another user cannot be designed to be kept at a sufficiently low level, when the number K of users increase. A pilot-response pk is influenced by a multipath channel gain between transceivers in addition to the spreading sequence, and an inter-user cross-correlation between a pair of such pilot responses takes a larger value than the correlation between the corresponding spreading-sequences themselves. Furthermore, the multipath delayed waves of an adjacent symbol generate an inter-symbol interference (ISI), which impedes an increase in the number K of users. Hance, it is impossible to improve the frequency-utilization-efficiency.
In order to suppress disturbance caused by the above-described interfering noise, many methods for multi-user receivers which perform user separation and adjacent symbol separation by solving decorrelating equations have been studied. However, a sufficient noise suppression effect has not been obtained. Here, a list of seven preceding techniques closely related to the present invention is shown below.
(A) [Mamoru Sawahashi, Yoshinori Miki, Hidehiro Andoh and Kenichi Higuchi, Pilot Symbol-Assisted Coherent Multistage Interface Canceller Using Recursive Channel Estimation for DS-CDMA Mobile Radio” IEICE Trans. Commun., Vol.E79-B, No. 9, pp. 1262 to 1270, (09.1996)]
(B) [Mitsuhiro Tomita, Noriyoshi Kuroyanagi, Satoru Ozawa and Naoki Suchiro, “Error rate performance improvement for a de-correlating CDMA receiver by introducing additional dummy pilot response”, PIMRC '02, Lisbon (09.2002)]
(C-1) [D. Koulakiotis and A. H. Aghvami, CTR, King's Collage, University of London “Data Detection Techniques for DS-CDMA Mobile Systems: A Review”, IEEE Personal Comm., pp. 24 to 34, June 2000]
(C-2) [T. Abe and T. Matsumoto, “Space-Time Turbo Equalization and Symbol Detection in Frequency Selective MIMO Channels with Unknown Interference”, Proc. WPMC '01, Aalborg Denmark (09.2001)]
(D) [Siavash M. Alamouti “A Simple Transmit Diversity Technique for Wireless Communications” IEEE JSAC, Vol. 16, No. 8 (10.1998)]
(E) [Naoki Seuhiro, Noriyoshi Kuroyanagi, Toshiaki Imoto and Shinya Matsufuji, “Very Efficient Frequency Usage Systems using Convolutional Spread Time Signals Based on Complete Complementary Code”, PIMRC '2000, (09.2000)]
(F) [Jiangzhou Wang and Jun Chen “Performance of Wideband CDMA Systems with Complex Spreading and Imperfect Channel Estimation” IEEE JSAC Vol. 19, No. 1, (01.2001)]
System (A) intends to upgrade the function of the k-th matched filter MFk to detect a data of the k-th user uk in the system explained with FIG. 16, and uses a receiver equipped with interference cancellers shown in FIG. 17. In an interference canceller IC-1 (the first stage), a matched filter bank MFB generates estimated transmit-data (soft-outputs) {tilde over (b)}[k] of all the users except that of the (k1)-th user by using a received input r1 and a pilot-response supplied from a pilot response memory PRM. By using soft-outputs {tilde over (b)}[k], the first interference generator I-GEN1 generates a replica (pseudo input) Φ[k]. By subtracting Φ[k] from input r1, interference canceller IC-1 generates a soft-output {tilde over (b)}k1. By making a hard decision on soft-output {tilde over (b)}k1, is obtained a detected value {circumflex over (b)}k1, with which a corresponding replica Φk1 is generated with the second interference generator I-GEN2. To a canceller (called the second stage) IC-2, is applied an input r2 which is made by subtracting replica Φk1 from received input r1. Canceller IC-2 repeats to apply the same operation to input r2 as that IC-1 has done. In this method, due to existence of large cross-correlations between pilot-responses of respective users, large interference components resultantly remain in the soft-outputs. For this reason, an error rate cannot be sufficiently reduced.
FIG. 18 shows a functional block diagram of a multi-user receiver. FIG. 18(a) shows a De-correlating Detector (system DD) corresponding to System (B). In this case, each user transmitter transmits a pilot symbol, so as not to be disturbed by interfering waves from the other users. The receiver receives these pilot symbols and always prepares highly accurate pilot-responses from all the users in a pilot response memory PRM. Each user transmitter uses both a sequence allocated to the user, and a common carrier-wave used by all the users to generate a transmit-data-symbol to be transmitted by the transmitter. More specifically, in System (B), each user transmits a pilot-signal to a base-station (BS) so that BS can accurately recognize channel-gain-characteristics (channel) from each user to the base-station. Therefore, base-station BS can obtain a pilot-response (channel-gain-property) pk of a transmission-paths from the k(=1, 2 . . . , K)-th user uk. A receive-symbol r is given by the following equation:
                    r        =                                            ∑                              k                =                0                            K                        ⁢                                                  ⁢                                          b                k                            ⁢                              P                k                                              +          x                                    (                  A          ⁢                      -                    ⁢          1                )            where bk is a transmit-data of the k-th user uk, and x is white noise (AWGN) included in receive-symbol r. By using a pilot-response-matrix P consisting of pilot-responses pk of all the users, Eq.(A-1) is solved by a de-correlating detector (Decor) in FIG. 18 to obtain a soft-output {tilde over (b)}k=bk+Δbk corresponding to the transmit-data where Δbk is an error. This system has an advantage such that the influence of interfering waves can be completely removed.
However, in System (B), since pilot-response-matrix P is dependent on a channel gain, the regularity of matrix P often decreases, and the AWGN component is amplified in a process of solving the equation, resulting in an increase in an error Δbk contained in the soft-output. Therefore, the number of users K and the spreading-sequence length L must satisfy a relationship: K<<L to reduce error Δbk. More specifically, there are problems such that the number of users to be accommodated is limited and the system is forced to have a low frequency-utilization-efficiency.
Systems (C-1) and (C-2) use a Minimum-Mean-Square-Error Detector (MMSE-D) shown in FIGS. 18(b) and 18(c). Although system MMSE-D provides a method to solve the same de-correlating equations as solved by system DD to generate soft-outputs, in order to increase the regularity of matrix P, it modifies matrix P with an additive term in the solving process. Due to the additive term, interfering noise is generated. Since the interfering noise decreases signal components contained in a receive-vector, the quality of the soft-output are degraded. System (C-2) provides a method to overcome a drawback of the MMSE-D. Consider the k-th user uk as a target user. A soft-output-vectors {tilde over (b)} consisting of all the user components is calculated by a conventional (as the first stage) MMSE, and an estimated received input (replica) φ[k] received from users except the k-th user uk is calculated from a channel matrix H composed of channel gains of all the users estimated from the pilot-response P, and a soft-output-vectors {tilde over (b)}[k] which is made by removing the k-th user's soft-output component from soft-output-vector {tilde over (b)}, The receiver calculates the second stage receive-symbol rk by removing replica φ[k] that is an interference component for the k-th user uk, from the first stage receive-symbol r1. Symbol rk is applied to a conventional (as the second stage) MMSE, to obtain the k-th soft-output {tilde over (b)}k. The system makes hard decision on soft-output {tilde over (b)}k to produce the k-th detected value {circumflex over (b)}k. Since soft-output-vector {tilde over (b)}[k] does not include the k-th soft-output {tilde over (b)}k, an interference component generated by the k-th data bk cannot be removed in this process. Therefore, since replica φ[k] includes large interfering noise, an improvement in frequency efficiency is not sufficient.
System (D) deals with a single-user system where space-time coded transmission is performed on a multiple-input multiple-output (MIMO) system. In the system, by using a plurality (N) of transmit-antennae as a space axis and a plurality (Nτ) of symbol slots as a time axis, N data are transmitted using NNτ symbols to get an advanced space-time diversity effect. In order to design respective transmit-symbol sets each consisting of N symbols simultaneously to be transmitted so as to have orthogonalities each other, a transmitter multiplies the respective of N transmit-symbols of the respective combinations by respective element sequences (code-words) of an orthogonal sequence-set (code) so that a receiver easily can separate the N data from each other. An effective technique such as to apply this method to a multi-user receiver has not been established. Furthermore, in the system, since this system assumes that the channel characteristics during a time for transmitting Nτ symbols are invariant, a sufficient diversity effect on the time axis cannot be obtained.
System (E) utilizes such a characteristic that complementary sequences have orthogonalities at all shift positions. A user of this system produces a synthesized transmit-symbol by adding plural element sub-symbols, each is made so as to multiply one of the sequentially chip shifted complementary sequences by transmit-data and transmits the synthesized transmit-symbol. Since this system can transmit a large number of element sub-symbols over a symbol period, frequency-utilization-efficiency is improved. Furthermore, the receiver can easily separate and discriminate the plural data carried on sub-symbols because the shifted sequences are orthogonal each other. However, since a sum of a large number of the element sub-symbols is transmitted, there is a problem such that a peak transmit-power considerably increases.
System (F) is a pilot-data multiplexed transmission system standardized as one of the 3rd Generation systems in which a user transmits simultaneously a pilot-symbol and a data-symbol. In order to separate a data and a pilot at a receiver, this system sets a real number as data (b∈±1) and an imaginary number a pilot (p=j). The same spreading-sequence is modulated by a complex number (b+j) to make a transmit-symbol. More specifically, since an 1-bit data/symbol is transmitted by using a symbol time slot and a band-width in which a 2-bit/symbol can be transmitted by QPSK (4-phase shift keying modulation), the pilot consumes considerably large resources equal to that the data dose. Furthermore, since a pilot is subjected to interferences by both a data symbol from the same user and pilot/data-symbols from the other users due to the multi-path channel characteristics, there is also a problem such that an accurate pilot-response cannot be easily obtained.